id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
22293	"ERROR: relation ""app_taggedcountryoforigin"" already exists"	Daniel Hahler	nobody	"There is a problem in db.backends.schema.alter_field, with the call to `_alter_many_to_many`:

it might try to rename a DB table to the same name.

With django-taggit, changing arguments for the TaggableManager a migration is created, which triggers a RENAME of a DB table, although the name is not changed (e.g. when changing `blank`).

The migration:

    operations = [
        migrations.AlterField(
            model_name='model',
            name='country_of_origin',
            field=taggit.managers.TaggableManager(through=app.models.TaggedCountryOfOrigin, blank=True, help_text=u'A comma-separated list of tags.', verbose_name=u'Country of origin'),
        ),

The SQL:

{{{
    % manage.py sqlmigrate app 0010
    ALTER TABLE ""app_taggedcountryoforigin"" RENAME TO ""app_taggedcountryoforigin"";
    ALTER TABLE ""app_taggedcountryoforigin"" DROP CONSTRAINT app__tag_id_5f04ec758472f8f8_fk_app_countryoforigintag_id;
    ALTER TABLE ""app_taggedcountryoforigin"" ADD CONSTRAINT app_taggedcountryoforigin_tag_id_5f04ec758472f8f8_fk FOREIGN KEY (""tag_id"") REFERENCES ""app_countryoforigintag"" (""id"") DEFERRABLE INITIALLY DEFERRED;
}}}

The SQL error: ERROR: relation ""app_taggedcountryoforigin"" already exists


The code path is as follows:

ipdb> l
    128                 elif isinstance(to_field.rel.to, six.string_types):
    129                     to_field.rel.to = from_field.rel.to
--> 130             schema_editor.alter_field(from_model, from_field, to_field)
    131 
    132     def database_backwards(self, app_label, schema_editor, from_state, to_state):
    133         self.database_forwards(app_label, schema_editor, from_state, to_state)
    134 
    135     def describe(self):
    136         return ""Alter field %s on %s"" % (self.name, self.model_name)
    137 
    138     def __eq__(self, other):

ipdb> from_model
<class '__fake__.Model'>
ipdb> from_field
<taggit.managers.TaggableManager: country_of_origin>
ipdb> to_field
<taggit.managers.TaggableManager: country_of_origin>
ipdb> 


> …/django-master/django/db/backends/schema.py(767)_alter_many_to_many()
    766         # Rename the through table
--> 767         self.alter_db_table(old_field.rel.through, old_field.rel.through._meta.db_table, new_field.rel.through._meta.db_table)
    768         # Repoint the FK to the other side


The code that calls it (from `database_forwards`):

        if router.allow_migrate(schema_editor.connection.alias, to_model):
            from_field = from_model._meta.get_field_by_name(self.name)[0]
            to_field = to_model._meta.get_field_by_name(self.name)[0]
            # If the field is a relatedfield with an unresolved rel.to, just
            # set it equal to the other field side. Bandaid fix for AlterField
            # migrations that are part of a RenameModel change.
            if from_field.rel and from_field.rel.to:
                if isinstance(from_field.rel.to, six.string_types):
                    from_field.rel.to = to_field.rel.to
                elif isinstance(to_field.rel.to, six.string_types):
                    to_field.rel.to = from_field.rel.to
            schema_editor.alter_field(from_model, from_field, to_field)

(This gets triggered by my patch to taggit (http://github.com/blueyed/django-taggit/compare/rel.through._meta.auto_created-for-1.7?expand=1), where I setup `auto_created`.

I came up with this in an attempt to fix another taggit issue with Django 1.7 (https://github.com/alex/django-taggit)/issues/211 - ""changed `through` model: ValueError: Cannot alter field ... - they are not compatible types"".)

I will provide a fix for this, but did not manage to come with a test case (for the test suite).

I would really appreciate it, if 
"	Bug	closed	Migrations	dev	Normal	fixed			Accepted	1	0	0	0	0	0